Pull swimmer back downwind (7)
I believe the answer is:
leeward
'downwind' is the definition.
('leeward' can be similar in meaning to 'downwind')
'pull swimmer back' is the wordplay.
'pull' becomes 'draw' (draw is a kind of pull).
'swimmer' becomes 'eel' (eel is a kind of aquatic vertebrate).
'back' shows that the letters should be reversed in order.
'draw'+'eel'='draweel'
'draweel' written backwards gives 'LEEWARD'.
(Other definitions for leeward that I've seen before include "away from the weather" , "to one side of boat" , "On the downwind side" , "Of ship, on side turned away from wind" , "Quarter towards which the wind blows" .)